The invention relates to thermoplastic molding compositions comprising                A) from 29 to 99.7% by weight of a thermoplastic polyester,        B) from 0.3 to 3% by weight (based on 100% by weight of A)+B)) of an imide salt of the formula        
                                    or a mixture of these,                        where Me is intended to be an alkali metal,        C) from 0 to 70% by weight of further additives,where the total of the percentages by weight of A) to C) is 100%.        
The invention further relates to the use of the molding compositions for producing fibers, foils, and moldings, in particular laser-transparent moldings, and for producing moldings by means of laser transmission welding processes, the processes for producing moldings of this type, and also to the use of these in various application sectors.
Components B) of this type are by way of example mentioned in EP 021648 and EP 2022826 as nucleating agents for compounded PET materials. The optical properties of the compounded materials were not investigated.
There are various processes (Kunststoffe 87, (1997), 11, 1632-1640) for welding plastics moldings. In the case of the widely used processes of heated-tool welding and vibration welding (e.g. of motor-vehicle inlet manifolds), precondition for a stable weld is adequate softening of the adherends in the contact zone prior to the actual step that produces the join.
Laser transmission welding is a method providing an alternative to vibration welding and heated-tool welding, and has seen a constant increase in its use in recent times, in particular with use of diode lasers.
The technical literature describes the fundamental principles of laser transmission welding (Kunststoffe 87, (1997) 3, 348-350; Kunststoffe 88, (1998), 2, 210-212; Kunststoffe 87 (1997) 11, 1632-1640; Plastverarbeiter 50 (1999) 4, 18-19; Plastverarbeiter 46 (1995) 9, 42-46).
Precondition for the use of laser transmission welding is that the radiation emitted from the laser first passes through a molding which has adequate transparency for laser light of the wavelength used, and which in this patent application is hereinafter termed laser-transparent molding, and is then absorbed, in a thin layer, by a second molding which is in contact with the laser-transparent molding and which hereinafter is termed laser-absorbent molding. Within the thin layer that absorbs the laser light, the energy of the laser is converted into heat, which leads to melting within the contact zone and finally to bonding of the laser-transparent and of the laser-absorbent molding via a weld.
Laser transmission welding usually uses lasers in the wavelength range from 600 to 1200 nm. In the wavelength range of the lasers used for thermoplastics welding, it is usual to use Nd:YAG lasers (1064 nm) or high-power diode lasers (from 800 to 1000 nm). When the terms laser-transparent and laser-absorbent are used hereinafter, they always refer to the abovementioned wavelength range.
A requirement for the laser-transparent molding, in contrast to the laser-absorbent molding, is high laser transparency in the preferred wavelength range, so that the laser beam can penetrate as far as the weld area, with the necessary energy. By way of example, transmittance for IR laser light is measured by using a spectrophotometer and an integrating photometer sphere. This measurement system also detects the diffuse fraction of the transmitted radiation. The measurement is carried out not merely at one wavelength but within a spectral range which comprises all of the laser wavelengths currently used for the welding procedure.
Users presently have access to a number of laser-welding-process variants based on the transmission principle. By way of example, contour welding is a sequential welding process in which either the laser beam is conducted along a freely programmable weld contour or the component is moved relatively to the immovable laser. In the simultaneous welding process, the linear radiation emitted from individual high-power diodes is arranged along the weld contour to be welded. The melting and welding of the entire contour therefore takes place simultaneously. The quasi-simultaneous welding process is a combination of contour welding and simultaneous welding. Galvanometric mirrors (scanners) are used to conduct the laser beam at a very high velocity of 10 m/s or more along the contour of the weld. The high traverse rate provides progressive heating and melting of the region of the join. In comparison with the simultaneous welding process, there is high flexibility for alterations in the contour of the weld. Mask welding is a process in which a linear laser beam is moved transversely across the adherends. A mask is used for controlled screening of the radiation, and this impacts the area to be joined only where welding is intended. The process can produce very precisely positioned welds. These processes are known to the person skilled in the art and are described by way of example in “Handbuch Kunststoff-Verbindungstechnik” [Handbook of plastics bonding technology](G. W. Ehrenstein, Hanser, ISBN 3-446-22668-0) and/or DVS-Richtlinie 2243 “Laserstrahlschweiβen thermoplastischer Kunststoffe” [German Welding Society Guideline 2243 “Laser welding of thermoplastics”].
Irrespective of the process variant used, the laser welding process is highly dependent on the properties of the materials of the two adherends. The degree of laser transparency (LT) of the transparent component has a direct effect on the speed of the process, through the amount of energy that can be introduced per unit of time. The inherent microstructure, mostly in the form of spherulites, of semicrystalline thermoplastics generally gives them relatively low laser transparency. These spherulites scatter the incident laser light to a greater extent than the internal structure of a purely amorphous thermoplastic: back-scattering leads to a reduced total amount of transmitted energy, and diffuse (lateral) scattering often leads to broadening of the laser beam and therefore to impaired weld precision. These phenomena are particularly evident in polybutylene terephthalate (PBT), which in comparison with other thermoplastics that crystallize well, such as PA, exhibits particularly low laser transparency and a high level of beam expansion. PBT therefore continues to be comparatively little used as material for laser-welded components, although other aspects of its property profile (e.g. good dimensional stability and low water absorption) make it very attractive for applications of this type. Although semicrystalline morphology is generally unhelpful for high laser transparency, it provides advantages in terms of other properties. By way of example, semicrystalline materials continue to have mechanical strength above the glass transition point and generally have better chemicals resistance than amorphous materials. Materials that crystallize rapidly also provide processing advantages, in particular quick demoldability and therefore short cycle times. It is therefore desirable to combine semicrystallinity with rapid crystallization and high laser transparency.
There are various known approaches to laser-transparency improvement in polyesters, in particular PBT. In principle, these can be divided into blends/mixtures and refractive-index matching.
The approach using blends/mixtures is based on “dilution” of the low-laser-transparency PBT by using a high-laser-transparency partner in the blend/mixture. Examples of this are found in the following specifications: JP2004/315805A1 (PBT+PC/PET/SA+filler+elastomer), DE-A1-10330722 (generalized blend of a semicrystalline thermoplastic with an amorphous thermoplastic in order to increase LT; spec. PBT+PET/PC+glass fiber), JP2008/106217A (PBT+copolymer with 1,4-cyclohexanedimethanol; LT of 16% increased to 28%). A disadvantage here is that the resultant polymer blends inevitably have properties markedly different from those of products based predominantly on PBT as matrix.
The refractive-index matching approach is based on the different refractive indices of amorphous and crystalline PBT, and also of the fillers. By way of example, comonomers have been used here: JP2008/163167 (copolymer of PBT and siloxane), JP2007/186584 (PBT+bisphenol A diglycidyl ether) and JP2005/133087 (PBT+PC+elastomer+high-refractive-index silicone oil) may be mentioned as examples. Although this leads to an increase in laser transparency, this is achieved with loss of mechanical properties. The refractive-index difference between filler and matrix can also be reduced, see JP2009/019134 (epoxy resin coated onto glass fibers in order to provide matching at the optical interface between fiber and matrix), or JP2007/169358 (PBT with high-refractive-index glass fiber). Starting materials of this type are, however, disadvantageous because of their high costs and/or the additional stages that they require within the production process.
The effects achieved in relation to laser-transparency increase are also overall relatively minor and therefore not entirely satisfactory.